Optical system for endoscope

Abstract: A relay lens system to be incorporated in an endoscopic optical assembly is provided. The relay lens system includes at least one relay lens module 42 for relaying an image between successive image planes. The relay lens module consists of two identical optical assemblies 44 arranged in bilateral symmetrical relation relative to a median plane disposed between the two assemblies. Each optical assembly includes a glass plano cylinder 46 and first 48 and second 50 curved lens components attached to each end of the plano cylinder. The lenses forming the lens components are fabricated from different optical materials including polymeric materials to correct aberrations within the system. The physical and geometrical characteristics of the components are selected to effectively transfer a bright image to the viewer.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to endoscopic optical systems and, in particular, to a relay lens system for a disposable rigid endoscope. 2. Discussion of the Prior Art Endoscopes are optical instruments which permit the examination of body cavities without the need for extensive surgery. Conventional rigid endoscopes typically include a pipe for illuminating a region of the body cavity to be viewed and an optical lens system mounted in a tube for focusing and relaying the illuminated image from inside the body cavity to the physician. Essential optical components of a conventional lens system include a front combination of lenses constituting the objective lens, a system of relay lenses to carry the image through the scope and an eye lens which produces a magnified virtual image for the viewer. Examples of such optical systems for endoscopes are described in U.S. Patent Nos. 3,089,484 to Hett, 3,257,902 to Hopkins, 3,556,085 to Takahashi, 4,036,218 to Yamashita, 4,267,828 to Matsuo and 4,273,110 to Groux. There are several drawbacks in the endoscopes of the prior art, specifically with regard to the complexity and expense of the optical systems incorporated therein. The optical components are typically made of glass that is ground and polished by expensive manufacturing techniques. As a result, the cost of these instruments is relatively high which thereby precludes disposing of the instrument after each surgery. It has become increasingly important to dispose of these instruments after each surgery to eliminate the risk of exposing the next patient to diseases such as AIDS or hepatitis. While most surgical instruments can be sterilized with high pressure steam, the delicate nature of optical systems in endoscopes makes such sterilization difficult. Also, since most of these instruments are inherently fragile, they are frequently broken, at great cost and inconvenience to the user. In an effort to reduce the cost and complexity of the prior art systems, U.S. Patent No. 4,784,118 to Fantone describes an endoscope in which the light pipe, and the objective, relay, and viewing lens assemblies are all made of polymeric materials, such as acrylics, polystyrenes, polycarbonates and styrene-acrylonitrile (SAN) copolymers, by conventional injection molding techniques. Although the Fantone device offers some advantages over prior art optical systems, the device has several drawbacks. The polymeric relay lenses are composed of only one material which increases the chromatic aberration of the image and reduces the resolution. In addition, in order to obtain a bright image, the polymeric lenses have to be manufactured such that the length to diameter ratios of many of the lenses is relatively high. This is very difficult to achieve using currently known manufacturing techniques. U.S. Patent No. 4,964,710 to Leiner discloses a unique hybrid relay lens system that overcomes the problems with the Fantone endoscope and the other aforementioned prior art. This relay lens system incorporates glass plano cylinders disposed between molded polymeric curved surface lenses which have a thickness on the same order of magnitude as their diameter. The polymeric lenses are optionally fabricated from two different polymers to facilitate correction of chromatic aberration. In order to achieve a bright image, the plano glass cylinders with flat polished end faces are placed in between the polymeric lenses. In contrast to ground and polished lenses, the plano glass cylinders can be economically made in large quantities, while the smaller polymeric lenses can be economically and accurately made by known injection molding processes. Despite the teachings of the prior art, it remains desirable to provide an endoscope which is efficient and reliable to manufacture and which provides a brighter image while enhancing resolution. The present invention relates to a device which achieves these objectives by providing an improved relay lens system to be incorporated in a rigid endoscope. WO89/11112 describes an endoscope relay optics design with a central rod-like element of high index material, a pair of negative elements of relatively low index material cemented to opposite ends of the central element, and a pair of positive end elements of high index material cemented to the negative elements. Thus, this described device is an assembly which has symmetry across a plane halfway along the length of the central element. The features of the preamble of claim 1 are, in combination, known from WO 89/11112. SUMMARY OF THE INVENTION According to the claimed invention, a relay lens system using a combination of polished glass and molded polymeric lenses is provided to be incorporated in an endoscope. The relay lens system comprises at least one relay lens module for relaying an image between successive image planes within the endoscope. The relay lens module comprises at least one plano glass cylinder and at least one polymeric lens component axially aligned with the plano glass cylinder and adhered to an end surface thereof. The relay lens module comprises two lens assemblies arranged in bilateral symmetrical relation about a median plane disposed between the two assemblies. Each lens assembly comprises a central glass piano cylinder having first and second flat end surfaces, a first lens component axially aligned with the piano cylinder and positioned adjacent the first end surface thereof, this end surface being arranged remote from the median plane, and a second lens component with at least two lenses and being axially aligned with the piano cylinder and positioned adjacent the second end surface thereof, this end surface being arranged adjacent the median plane. The first lens component functions as a field lens. At least one of the first and second lenses is adjacent a respective end surface of the central glass piano cylinder. This adjacent lens as well as the first lens component are fabricated from a polymeric material. Preferably, the relay lens system of the present invention comprises a plurality of identical lens modules aligned along a common axis within the endoscope. The geometrical and physical characteristics of each module are selected to effectively transfer a bright image of an object to a successive image plane while also allowing for the correction of image aberrations. It is envisioned that the relay lens system can be readily incorporated into conventional endoscopic instrumentation and used in combination with conventional objective lens and eye lens assemblies. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, referred to herein and constituting a part hereof, illustrate preferred embodiments of the present invention and together with the description, serve to explain the principles of the present invention, wherein: FIG. 1 is an optical schematic of an endoscopic optical system incorporating the relay lens system of the present invention and illustrating ray path and image orientation; FIG. 2 is an enlarged optical schematic of a single relay lens module of the relay lens system shown in FIG. 1 and illustrates ray path and image orientation within the module; FIG. 3 is an enlarged optical schematic of an optical assembly of the single relay lens module of FIG. 2; FIG. 4 is an enlarged optical schematic of an alternative single relay lens module to be incorporated in an endoscopic optical system and illustrates ray path and image orientation; FIG. 5 is an enlarged optical schematic of another alternative single relay lens module to be incorporated in an endoscopic optical system and illustrates ray path and image orientation; and FIG. 6 is a general graphic representation of modulation transfer function (MTF) curves for the above embodiments of the relay lens systems. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings and, in particular, to FIG. 1, there is shown a schematic of an endoscopic optical system 20 incorporating the novel relay lens system 40 of the present invention. Relay lens system 40 finds particular application in disposable endoscopes and can be used in combination with a conventional objective lens assembly 60 and a conventional eye lens assembly 80 as shown. Relay lens system 40 includes a plurality of relay lens modules 42, arranged in end to end fashion along a common axis. Each module 42 is identical with regard to the optical elements contained therein, and is capable of transferring an image from an image plane at the entrance side of the module to a successive image plane formed on the exit side. Referring now to FIG. 2, the relay lens module 42 of the present invention is illustrated in detail. Lens module 42 includes two identical optical assemblies 44 arranged in a bilateral symmetrical end to end manner about a median plane equidistantly disposed between the two assemblies. The assemblies 44 are separated by an air gap. Each assembly 44 includes a glass plano cylinder 46 having a polished adjacent end face and a polished outer end face relative to the other component in the module. Glass plano cylinder 46 ensures the transfer of a bright image between the modules. Each assembly 44 further includes single lens 48 and doublet lens 50 bonded to the outer end face and the adjacent end face of plano cylinder 46, respectively. As best shown in FIG. 3, single lens 48 is preferably a plano-convex lens with the planar surface of the lens being bonded to plano cylinder 46 by conventional adhesive means. A suitable adhesive for this purpose is Norland 68 manufactured by Norland Products Inc. The convex surface of lens 48 may be coated with a broad band anti-reflection coating to reduce reflection losses at the air-lens interface. Single lens 48 is fabricated from a polymeric material, such as an acrylic, polystyrene, polycarbonate, or copolymer styrene-acrylonitrile (SAN). In a preferred embodiment, single lens 48 is fabricated from an acrylic. Single lens 48 may be formed by conventional injection molding techniques, such techniques being capable of forming precise lenses, relatively inexpensively and in large volumes. Doublet lens 50 consists of two lens elements, specifically, first lens 52 and second lens 54, bonded to each other along adjacent surfaces to form the doublet. Preferably, first lens 52 is a plano-convex lens with the planar surface of the lens 52 being bonded to the adjacent end face of cylinder 46. Second lens 54 is preferably a meniscus (concavo-convex) lens. Doublet 50 is preferably fabricated from two different polymeric materials to allow for the correction of chromatic aberration within the lens system. In a preferred embodiment, lens 52 is formed from an acrylic while lens 54 is formed from either a polycarbonate or polystyrene. It is to be noted that other combinations of optical materials for correcting chromatic aberration would be apparent to one skilled in the art. Lens 52 and 54 may also be manufactured by known injection molding techniques. In the optical arrangement of lens module 42, single lens 48 functions as a field lens, and as such, bends light ray bundles at the edge of the field, which would otherwise miss doublet lens 50, back toward the longitudinal axis defined by the optical system to thereby minimize vignetting. This is a significant feature of the present invention in that the field of view may be increased without increasing the diameter of doublet 50. Referring again to FIG. 2, the geometrical characteristics of module 42 are defined by an entrance image plane 45a, a front surface 45b, a first bonded surface 45c, a second bonded surface 45d, a third bonded surface 45e, a first inner surface 45f, a second inner surface 45g, a fourth bonded surface 45h, a fifth bonded surface 45i, a sixth bonded surface 45j, a rear surface 45k and an exit image plane 451. The geometrical and optical parameters of module 42 of the relay lens system 40 of the present invention are recorded in Table 1 below. In the Table, surfaces A, B-K and L correspond to image plane 45a, surfaces 45b-45k and exit image plane 451, respectively. SURFACE RADIUS THICKNESS MEDIUM INDEX ABBE NO. A Image Plane 1.0 ± .03 Air 1.000 B 13.615 3.0 ± .03 Acrylic 1.492 57.4 C Plano 44.0 ± .10 SF2 1.648 33.9 D Plano 3.0 ± .03 Acrylic 1.492 57.4 E 30.9 -10.860 1.0 ± .03 Polystyrene 1.590 F -15.770 1.0 ± .03 Air 1.000 G 30.9 15.770 1.0 ± .03 Polystyrene 1.590 H 57.4 10.860 3.0 ± .03 Acrylic 1.492 I 33.9 Plano 44.0 ± .10 SF2 1.648 J 57.4 Plano 3.0 ± .03 Acrylic 1.492 K -13.615 1.0 ± .03 Air 1.000 L Image Plane * dimensions are in millimeters While Table 1 provides a description of a preferred embodiment of the invention, various modifications, alternate construction materials and equivalents may be employed without departing from the spirit of the invention. For example, the size of the air gap between optical components may be altered as well as the radii of curvature of the lenses and type of polymeric lenses incorporated in the optical module. In a preferred embodiment, the relay lens system comprises three lens modules aligned in end to end fashion along a common axis as shown in FIG. 1. Preferably, the lens modules are interconnected by spacers (not shown). Interconnecting the modules is very desirable in that the relay lens assembly may be assembled as a single unit. This greatly facilitates mounting of the relay lens assembly within the endoscope. Each module 42 of the relay lens system of the present invention may be secured to a disposable endoscope by conventional methods and, as previously mentioned, may be used in combination with a conventional objective lens assembly 60 and eye lens assembly 80 as shown in FIG. 1 to effectively transfer an image from the object site to the viewer. Preferably, the objective lens and eye lens are fabricated from polymeric materials as well as to further reduce the cost of the endoscope. Referring now to FIG. 4 there is illustrated a schematic view of an alternative embodiment of a single relay lens module to be incorporated in an endoscopic optical system. Lens module 100 is substantially similar to the lens module described in connection with the embodiment of FIG. 2 and includes two lens assemblies 102 separated by an air gap. Each assembly includes a glass plano cylinder 104 having a single plano convex lens 106 affixed to a first end face thereof and a doublet lens 108 affixed to a second end face thereof. Single piano convex lens 106 may be formed of any suitable optical material. In a preferred embodiment, single lens 106 is formed from acrylic. Single lens 106 may also be formed of any of the aforementioned polymeric materials including polycarbonate, polystyrene and styrene-acrylonitrile (SAN) copolymers. Doublet lens 108 including first lens element 110 bonded to second lens element 112 may also be formed of any suitable optical material. In a preferred embodiment, first lens element 110 is a plano-concave lens and is fabricated from polystyrene. Second lens element 112 is a double convex lens and is fabricated from acrylic. The geometrical and optical parameters of this embodiment are recorded in Table 2 below. SURFACE RADIUS THICKNESS MEDIUM INDEX ABBE NO. A Image Plane 3.0 ± .03 Air 1.000 B 7.2000 3.0 ± .03 Acrylic 1.492 57.4 C Plano 10.0 ± .10 F2 1.620 36.4 D Plano 1.0 ± .03 Polystyrene 1.590 30.9 E 4.77317 3.0 ± .03 Acrylic 1.501 56.4 F -8.25904 4.0 ± .03 Air 1.000 G 8.25904 3.0 ± .03 Acrylic 1.492 57.4 H -4.77317 1.0 ± .03 Polystyrene 1.590 57.4 I Plano 10.0 ± .10 F2 1.620 36.4 J Plano 3.0 ± .03 Acrylic 1.492 57.4 K -7.2000 3.0 ± .03 Air 1.000 L Image Plane * dimensions are in millimeters The above-described embodiments provide an inexpensive relay lens system which furnishes a bright image to the viewer while also minimizing image aberrations. While many relay systems comprise as many as ten elements, there are only four elements in each lens module. The glass plano cylinders can be economically manufactured in large quantities. The polymeric lenses can be made with great precision and with economy of manufacture by known injection molding techniques. Referring now to FIG. 5 there is illustrated another alternative example of a relay lens system useful for understanding the present invention. Relay lens module 120 includes two relay lens assemblies 122 arranged in bilateral symmetrical end to end relation relative to a median plane disposed between the two assemblies 122 and separated by an air gap. Each assembly 122 includes a central glass plano cylinder 124 with first and second end surfaces, 126, 128 respectively. Adjacent first end surface 126 of central cylinder 124 is a first optical component 130 including a glass piano cylinder element 132 and a plano-concave lens 134 affixed to one end surface of the cylinder element. In the preferred embodiment, the plano-concave lens 134 is proximate central glass plano cylinder 124 and is separated from the central cylinder by an air gap. Lens 134 is preferably formed from acrylic. A second optical component 136 is affixed to the second end surface of central cylinder 124. Second lens component 136 is a doublet lens including a plano-concave lens 138 fabricated from polystyrene and a double convex lens 140 fabricated from acyclic. A significant feature of relay lens module 120 depicted in FIG. 5 is that the images formed by the module 120 are located within glass cylinder 132. Accordingly, in-focus blemishes are reduced as compared to systems in which the images are located near a lens surface. Referring still to FIG. 5, the geometrical characteristics of module 120 are defined by front surface 125a, a first bonded surface 125b, a second surface 125c, a third surface 125d, a fourth bonded surface 125e, a fifth bonded surface 125f, a sixth surface 125g, a seventh surface 125h, an eighth bonded surface 125i, a ninth bonded surface 125j, a tenth surface 125k, an eleventh surface 1251, a twelfth bonded surface 125m and a rear surface 125n. The geometrical and optical parameters of this embodiment of the module are recorded in Table 3 below. SURFACE RADIUS THICKNESS MEDIUM INDEX ABBE NO. A Plano 5.0 ± .03 F2 1.620 36.4 B Plano 3.0 ± .03 Acrylic 1.492 57.4 C -25.000 3.0 ± .03 Air 1.000 D Plano 10.0 ± .10 F2 1.620 36.4 E Plano 1.0 ± .03 Polystyrene 1.590 30.9 F 7.000 3.0 ± .03 Acrylic 1.492 57.4 G -8.000 6.0 ± .03 Air 1.000 H 8.000 3.0 ± .03 Acrylic 1.492 57.4 I -7.000 1.0 ± .03 NOR61 1.590 30.9 J Plano 10.0 ± .10 F2 1.620 36.4 K Plano 3.0 ± .03 Air 1.000 L 25.000 3.0 ± .03 Acrylic 1.492 57.4 M Plano 5.0 ± .03 F2 1.620 36.4 N Plano * dimensions are in millimeters The embodiment depicted in FIG. 5 shows only one relay lens module 120, but, as would be readily apparent to one skilled in the art that a practical endoscope may consist of several relay pairs. For example, an endoscope would typically consist of three lens modules 120. Further, the thickness of plano cylinder 132 may be twice the thickness shown in FIG. 5 since adjacent lens modules would be present in the optical system. In addition, an endoscope would also contain an objective lens system located distal to the relay lenses to provide a wide field of view and an eye lens system for magnifying the most proximal image coming out of the relay lens system. The relay lens systems of the three above-described embodiments are each highly effective in transferring an illuminated image of an object to the viewer. In particular, each lens system produces exceptional results with regard to modulation transfer function (MTF), which is a useful parameter in evaluating the optical quality or performance of an image forming system. MTF also referred to as frequency response, sine-wave response or contrast transfer, is commonly defined as the ratio of the modulation in the image to that in the object as a function of the frequency (cycles per unit angle or length) of the sine-wave distribution pattern. A plot of modulation against frequency (angular frequency of the bars expressed as so many lines per degree in the image space) is an applicable measure of the performance of an imaging system. It should be noted that, for a given lens, the plot of MTF differs by wavelength, by field obliquity, by orientation of the bars, and from point to point along the lens axis. FIG. 6 is a graph depicting MTF curves for the relay lens systems described above. MTF values were calculated along the lens axis and for points off the axis at 70 % of the maximum angular field of view. As illustrated in the graph, the system has high modulation at the low spatial frequencies as well as the higher spatial frequencies for these points. Specifically, the modulation for the spatial frequencies ranging from 0 cycles/degree to 6 cycles/degree remains above 0.9 for points along the lens axis and for points off the axis at 70 % of the maximum angular field of view. These modulation values are favorable when compared to conventional glass relay lens systems and indicate that the present lens system has theoretical optical resolution comparable to conventional glass relay lens systems. The claims which follow identify embodiments of the invention additional to those described in detail above.

Claims: A relay lens system (40) to be incorporated in an endoscopic optical assembly (20), said relay lens system (40) comprising: at least one relay lens module (42) for relaying an image between successive image planes (45a, 45l); said relay lens module (42) comprising two lens assemblies (44) arranged in bilateral symmetrical relation about a median plane disposed between the respective end faces (45f, 45g) of said two assemblies (44) ; each said lens assembly (44) comprising: a central glass plano cylinder (46) having a first flat end surface (45j; 45c) arranged remote from said median plane and a second flat end surface (45d; 45c) arranged adjacent said median plane; a first lens component (48) which is axially aligned with said glass plano cylinder (46), and is positioned adjacent said first flat end surface (45j; 45c); a second lens component (50) which is a doublet lens (50) including a first lens (52) and a second lens (54), is axially aligned with said glass plano cylinder (46), and is positioned adjacent said second flat end surface (45d; 45i); at least one of said first (48) and second (50) lens components (48, 50) being adhered to said respective flat end surface of said glass plano cylinder (46), characterised in that said first lens component is a single lens (48) which functions as a field lens and in that at least one of said field lens (48) and said first and second lens (52 ; 54) is fabricared from a polymeric material. A system as claimed in claim 1, wherein said field lens (48) comprises a polymeric material selected from acrylics, polystyrenes, polycarbonates and styrene-acrylonitrile (SAN) copolymers. A system as claimed in claim 1 or 2, wherein said field lens (48) is a curved lens. A system as claimed in any one of the preceding claims, wherein said field lens (48) is adhered to said first end surface (45j) of said central glass plano cylinder (46). A system as claimed in any one of claims 1 to 3, wherein said field lens (48) and said central glass plano cylinder (46) are separated by an air gap. A system of any one of the preceding claims, wherein said second lens component (50) comprises a polymeric material selected from acrylics, polystyrenes, polycarbonates and styrene-acrylonitrile (SAN) copolymers. A system as claimed in claim 6, wherein said second lens component (50) is adhered to said second end surface (45i) of said glass plano cylinder. A system as claimed in any one of the preceding claims, wherein said second lens component (50) comprises a doublet lens (52, 54). A system as claimed in any one of the preceding claims, wherein said lens assemblies (44) in said lens module (42) are separated by an air gap. An optical system for a rigid endoscope comprising: objective lens means (60) for producing an image of an object at a first image plane; a relay lens system as claimed in any one of the preceding claims; and eye lens means (80) for viewing an image formed at an exit image plane; wherein said optical system has a modulation for the spatial frequencies ranging from 0 cycles/degree to 6 cycles/degree remaining above 0.9 for points along the lens axis and for points off said axis at 70% of the maximum angular field of view.

Inventors: LEINER DENNIS C, LEINER, DENNIS C.
Applicants: MONADNOCK OPTICS INC, MONADNOCK OPTICS, INC.
Application year: 1994
Main classification: G02B23
IPC codes: G02B23, G02B13, A61B1